The present invention relates to a magnetic disk medium and a method of producing the same. Particularly, the invention relates to a magnetic disk medium in which the attraction between a magnetic head and the magnetic disk medium when the magnetic disk medium stops can be reduced and in which the circumferential magnetic characteristics thereof can be equalized, and a method of producing the same.
Heretofore, working for forming substantially concentrically worked traces 33 (hereinafter called "texture working") on surfaces of a substrate or on N-P-plated subbing films provided on the surfaces of the substrate as shown in FIG. 1 has been carried out in magnetic disk media, especially thin film disk media such as sputtered disks, plated disks and the like.
Generally, the carrying out of the texture working brings about the following effects:
(i) Attracting force due to a sucking phenomenon between a magnetic head and a magnetic disk medium when the magnetic disk medium stops is reduced;
(ii) Magnetic films formed on the texture-worked surfaces are made to have shape-anisotropy so that the circumferential magnetic characteristics thereof are equalized; and so on.
However, carrying out only the texture working has been not sufficient to reduce the attracting force when the disk medium stops.
The attracting force and the directional property which make the surface accuracy of the disk medium rough have the following relation.
For example, in the case where worked traces are formed in the radial direction of a magnetic disk medium, that is, in the direction perpendicular to the acting direction of the attracting force when the magnetic disk medium starts, the effect of reduction of the attracting force is less than the case where the worked traces are formed in the circumferential direction of the magnetic disk medium. In order to attain the same effect as the latter case, the disk surfaces must have average roughness R.sub.a not less than 100 nm. However, the value of this roughness will shorten the life of the CSS area of the disk medium.
Further, in the case where the texture working is made, the roughness of the texture working is antinomically limited as follows.
FIG. 3 is a graph view showing relations among floating height H.sub.o of a magnetic head from a magnetic recording disk medium when the magnetic head comes into contact with the rotating magnetic recording disk medium, attracting force F of the magnetic head acting on the magnetic recording medium when the magnetic recording medium stops, and average surface roughness R.sub.a of the magnetic recording medium.
In order to lower the floating height H.sub.o of the magnetic head when the magnetic head comes into contact with the rotating magnetic disk medium, it is necessary to reduce the surface roughness of the magnetic disk medium. For example, in the case where the floating height H.sub.o is established to be 0.2 .mu.m, it is necessary to make the average roughness R.sub.a substantially satisfy the condition R.sub.a .ltoreq.25 nm.
On the other hand, the attracting force F between the magnetic head and the magnetic disk medium when the disk medium stops becomes F.gtoreq.20 gf when the relation R.sub.a .ltoreq.25 nm is satisfied. Accordingly, a problem arises in that magnetic head bearing springs are deformed when the magnetic disk medium starts. For the purpose of eliminating the problem, the attracting force must have a value not larger than 5 gf, and in this condition, it is necessary to make the average surface roughness R.sub.a of the magnetic disk medium be not less than 50 nm. If the value of R.sub.a is so large as described above, the floating height of the head becomes too much.
As described above, it is difficult to establish the roughness of the texture working to be suited to the two values of F and H.sub.o.
Further, a method for preventing the attracting force has been proposed as described in Japanese Patent Unexamined Publication No. 54-23508. According to the proposed method, the plane accuracy of a contact start stop area 42 (hereinafter abbreviated to "CSS area") on the coating type magnetic disk medium is relatively lowered compared with that of a recording/reproducing area 41 to thereby reduce the attracting force between the head and the disk medium at the CSS area. However, the directional property for roughing the plane accuracy is not defined in the proposed method.
Further, the proposed method does not give any specific suggestion as to the relations among the attracting force F, the average roughness R.sub.a and the floating height H.sub.o, and does not give any suggestion as to a specific value of R.sub.a.